1. Field of the Invention
The present invention relates to a charging and discharging control circuit for controlling the charging and discharging of a secondary battery and to a charging type power supply device including the charging and discharging control circuit.
2. Description of the Related Art
A charging type power supply device using a secondary battery includes, in order to protect the secondary battery, a charging and discharging control circuit for detecting the overcharging and overdischarging of the secondary battery and an overcurrent flowing into a load to control the charging and discharging of the secondary battery. In order to protect the secondary battery and reduce current consumption, the charging and discharging control circuit has been designed in various ways and a circuit as described in JP 2002-238173 A has been proposed.
FIG. 7 shows a conventional charging type power supply device.
In the conventional charging type power supply device, a current flows from a secondary battery into a load 103 connected between external terminals 105 and 106 through a switching circuit 102. When a voltage at an overcurrent detection terminal 113 connected with the external terminal 106 becomes higher than an overcurrent detection voltage, a charging and discharging control circuit 210 controls to turn OFF the switching circuit 102. This state is referred to as overcurrent detection state.
In the overcurrent detection state, an N-channel transistor 251 of a pull-down circuit 219 and an N-channel transistor 252 of a switching circuit 220 are turned ON. Then, the overcurrent detection terminal 113 is pulled down to a VSS terminal 112 through a resistor 253. After the charging and discharging control circuit 210 enters the overcurrent detection state, when the load 103 is disconnected from the external terminals 105 and 106, a voltage at the overcurrent detection terminal 113 approaches a VSS voltage. When the voltage at the overcurrent detection terminal 113 becomes lower than the overcurrent detection voltage, the charging and discharging control circuit 210 is released from the overcurrent detection state to turn ON the switching circuit 102.
The above-mentioned circuit operation is referred to as automatic return operation. An impedance between the external terminals 105 and 106 at the time of automatic return is referred to as automatic return impedance.
In an overcharging state in which a charger 104 is connected between the external terminals 105 and 106 and a secondary battery 101 has a voltage higher than a predetermined voltage value, the charging and discharging control circuit 210 controls to turn OFF the switching circuit 102. This state is referred to as overcharging detection state.
In the overcharging detection state, the voltage at the overcurrent detection terminal 113 becomes lower than the VSS voltage by the charger 104. Therefore, the charging and discharging control circuit 210 controls to turn OFF the N-channel transistor 252 of the switching circuit 220, thereby preventing a charging current from flowing through the resister 253 and a parasitic diode 254 of a pull-down circuit 219.
However, the conventional charging type power supply device has a problem in that the current consumption is increased by a phenomenon as described below.
FIG. 8 is a cross sectional view showing the pull-down circuit and the switching circuit in the conventional charging and discharging control circuit 210. In the overcurrent detection state, the overcurrent detection terminal 113 is pulled up to a VDD terminal 111 through the external terminal 106, the load 103, and the external terminal 105. Therefore, there is the following problem. A base current flows from the overcurrent detection terminal 113 into a P-well of the N-channel transistor 252. Then, a parasitic bipolar transistor 501 is turned ON and a current flows from the VDD terminal 111 to the VSS terminal 112 through the resistor 253, thereby increasing the current consumption of the charging and discharging control circuit.
For the automatic return, it is necessary to reduce the base current to a value at which the parasitic bipolar transistor 501 is not turned ON. That is, it is necessary to increase the automatic return impedance. However, the switching circuit and the pull-down circuit as described above have a problem in that the calculation of the automatic return impedance is complicated because the automatic return impedance is nonlinearly changed by a voltage of the secondary battery 101.